Is it just my subjective perception, or is acceleration impacted by rain far more than braking and turning? Is it car dependent? Tire dependent? I drove a Spec Miata in SCCA Driving school this past weekend. In the dry I found that my usual GT3 slow-in/fast-out was extremely competitive. But I was gettting killed using the same approach the day it rained. It seemed no matter how much I slowed for a corner, I could barely touch the gas on exit. I only regained competitiveness by going into the corners faster and darn near coasting to the exit berm before I eased into the throttle. For context, I was driving on slick R888's in the dry and full tread R888's in the wet.

As an aside, I was surprised about how little car to car contact there was during three days of us rookies racing around Thunderhill. Lot's of folks went off however. In the last race in the rain, by the end it seemed the organizers got overwhelmed towing them out and gave up. I estimate 20% of the 37 car field DNF'd. The last laps you could barley pass anymore as there was a yellow in half the corners. I pretty much finished driving a parade lap waving at my buddies in the mud.

I drove Reno Fernley DE with R888 (3/32) in a cold, heavy downpour with my 996. Turning/acceleration were almost equally affected(lower mph) but the input application points were about the same, just consciously smoother being aware of the car's larger slip angles. Braking was affected the greatest for me. I had to brake much earlier, than in the dry, to avoid locking up the wheels. Part of this is my pad not getting up to temperature also.

If I was more ballsy (This was only a DE afterall), I would drive more like a slippery indoor karting track. Turnin earlier (set the frontend), feed the throttle to get the correct rotation and modulate the throttle to maintain the larger slip angles(not drifting) through the turns. It was much faster than trying to drive 'grip' style.

sidenote-
R888 are not rain tires, my street tires were faster on a wet track. At 75+ mph on the RF straights, I was hydroplaning and 'puckering' all the way. I tried driving to work on my R888 on the Bay Bridge @ 55mph and I was sliding around.

The miata has an incredible amount of roll stiffness - I'm guessing pound for pound, it's significantly stiffer in roll than your GT3. So, as David pointed out, you *really* need to disconnect at least your rear swaybar and preferably both swaybars for a "full rain" setup. Also, if you're running the typical proven sm setup of 0.5 to 0.6 degrees additional (neg) camber in the back compared to the front, your traction in the back will be further compromised since the car will not roll onto the tire.

I've found the 888 to be a fine rain tire, but I've found that is doesn't like the "traditional" lower pressures in the wet the same way that the RA1's do. I've found 36-38psi (on a spec miata) in the rain to be very good.